The present patent application claims the right of priority under 35 U.S.C. xc2xa7119 (a)-(d) and 35 U.S.C. xc2xa7365 of International Application No. PCT/EP00/03216, filed Apr. 11, 2000, which was published in German as International Patent Publication No. WO 00/64554 on Nov. 2, 2000, which is entitled to the right of priority of German Patent Application No. 199 18 728.2, filed Apr. 24, 1999.
The invention relates to a multi-stage, continuous process for evaporating polymer solutions, especially of thermoplastic polymers, by indirect heat exchange by passing the polymer solution through heat exchangers, especially tubular heat exchangers, having a downstream separator, and to an apparatus for carrying out that process. The invention relates also to polycarbonate treated by the process according to the invention having a residual bisphenol A content of  less than 5 ppm.
The removal of volatile components from a polymer solution is one of the last process steps in the preparation of many polymers. The volatile constituents to be removed may be either solvents and/or unpolymerised monomers. Depending on the order of magnitude of the viscosity of the polymer solution, different variants are known for removing the volatile components from the polymer solution; in each of those variants, the polymer solution is heated above the evaporation temperature of the volatile constituents. There are known as apparatuses, for example, thin-film evaporators, extruders and those having indirect heat exchange.
When heating the polymer solution it is crucial that the polymer should not be damaged thermally. Thermal damage causes undesirable colour changes or the presence of pinholes in the polymer.
EP-A-0 150 225, for example, describes an apparatus having two heat exchanger bundles connected in series. The heat exchanger bundles have rectangular channels disposed horizontally, in which the polymer solution is degassed. The apparatus is used mainly for the two-stage heating or cooling of viscous polymer solutions during the reaction, but its manufacture and operation are relatively complex.
EP-A1-0 451 602 discloses a process for concentrating polymer solutions, in which the preheated solution is, in a throttled manner, expanded into a helical flow tube and the concentrated solution is concentrated further in a second drying apparatus having self-cleaning elements which is located immediately downstream. The process has the disadvantage that the final concentration of solvent in the polymer is still comparatively high, and that the dwell time in the downstream drying apparatus is too long. Polymer particles are able to collect on the inner walls of the helical tube, reducing the quality of the product.
The object of this invention is, therefore, to make available a process and an apparatus for removing volatile components from a polymer solution, which process and apparatus do not exhibit the disadvantages of the processes known from the prior art.
That object could be achieved with the elaboration and provision of the process according to the invention, which is described in greater detail below. In addition, that object could be achieved with the elaboration and provision of the apparatuses according to the invention, which are described in greater detail below. In addition, that object could be achieved with the elaboration and provision of the polycarbonate according to the invention which has been treated by the process according to the invention and is described in greater detail below.
The invention provides a multi-stage, continuous process for evaporating polymer solutions, especially of thermoplastic polymers, by indirect heat exchange by passing the polymer solution through heat exchangers, especially tubular heat exchangers, having a downstream separator, characterised in that
a) first, in a single stage or in a plurality of individual stages, the polymer solution having a polymer content of from 5 to 20 wt. % is concentrated to from 60 to 75 wt. % in a combination of a tubular heat exchanger and a thin-film evaporator, or a coiled tube evaporator or in a tubular heat exchanger, each having a downstream separator, at a temperature of from 100 to 250xc2x0 C., the pressure in the separator being approximately from 0.1 to 0.4 MPa,
b) in a further stage, the polymer solution is concentrated from 60 to 75 wt. % to at least 95 wt. %, especially to more than 98 wt. %, in a tubular heat exchanger having a downstream separator, at a temperature of from 130 to 350xc2x0 C., the tubular heat exchanger having vertical, heated, straight tubes, with or without built-in static mixers, having an inside diameter of from 5 to 30 mm, preferably from 5 to 15 mm, and a length of from 0.5 to 4 m, preferably from 1 to 2 m, and the throughput per heat exchanger tube through the tubes being from 5 to 30 kg/h, preferably from 15 to 25 kg/h, based on the polymer, the pressure in the separator being from 3 kPa to 0.1 MPa, preferably from 5 kPa to 0.1 MPa, and an entrainer, especially carrier gas, being introduced into the delivery device located downstream of the separator, at the intake side, and
c) the degassed polymer is subsequently isolated and, optionally, granulated.
Nitrogen (N2) is preferably used as the entrainer. It has been found that, by introducing nitrogen, the residual content of volatile components can be reduced to  less than 500 ppm without the need for further measures in terms of apparatus.
The invention also provides an apparatus for evaporating polymer solutions, especially of thermoplastic polymers, by indirect heat exchange by passing the polymer solution through heat exchangers, especially tubular heat exchangers, having a downstream separator, characterised in that
a) the first stage or stages has or have a tubular heat exchanger and/or coiled tube evaporator (3) each having a downstream separator (4),
b) the further stage has a tubular heat exchanger (7) having a downstream separator (8), the tubular heat exchanger (7) having vertical, heated, straight tubes having an inside diameter of from 5 to 30 mm, preferably from 5 to 15 mm, and a length of from 0.5 to 4 m, preferably from 1 to 2 m, and
c) there is arranged downstream of each stage a delivery device (6, 9) for transporting the polymer solution, an entrainer, especially carrier gas, being introduced at the intake side of the delivery device of the further stage.
The invention also provides a polycarbonate treated by the process according to the invention, characterised by a residual bisphenol A (BPA) content of less than 5 ppm.